I have a Caran d'ache pen--not an expensive one, but one i like. It appears that the upper barrel is stuck, and the lower barrel has trapped the cartridge in its claws. I've tried with several jewelry tools to disengage the upper body, with no luck. It may have been put in wrong (not on its thread) when I put in a cartridge.

Any advice? A tool suggestion?

Thanks all -- I got it off. I fiddled it, and fiddled it and greased my fingers, and got it apart.